Unit 222 Measuring The Social World

SOC 222 -- MEASURING the SOCIAL WORLD
Session #6 -- CONFIDENCE INTERVALS
Oct 2013

TODAY’S OBJECTIVES

1. Understand what confidence intervals tell us
2. Understand how we get them
3. Know the SPSS procedure for getting confidence intervals

Terms to Know

Confidence interval
Mu (μ) t-curve normal curve

RICK’S SITUATION

Recap

His RQ:

Population:
Real population mean:

Sample:
Sample mean: 6

Sampling distribution: 2

Estimated mean = sample mean = 6

Estimated standard error =

CONFIDENCE INTERVALS

Way for rick to talk about how accurate he is
Accuracy in inferential stats

Confidence Interval Statements

Marginal error statements
Standardized scores calculating and converting Effect of Sample Size

More accurate population estimates
Narrow confidence interval
Political reporting

increase sample size

As N gets larger, the fraction gets smaller

As the fraction gets smaller, SE gets smaller

As SE gets smaller, the sampling distribution gets narrower

As the sampling distribution gets narrower, more samples become closer to the mean

So the best 95% of the samples are closer to the mean

So the confidence interval for these samples is closer to the mean

So the confidence interval shrinks, and population estimates get more precise and accurate.

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SPSS: Confidence Intervals

This procedure finds statistics about a single variable from a sample, and estimates confidence intervals for the variable.

1. Menu bar, Analyze, Descriptive Statistics, Explore
Opens the “Explore” Box
Variable list on left
Three working areas in the middle
Ignore the second and third
Option buttons on the right
A “Display” area underneath
Five action buttons on the